Determination of an Equilibrium Constant Keq Abstract The general purpose of this lab was to determine the equilibrium constant for a reaction containing a complex ion HSCN to produce FeSCN2 and to determine the equilibrium concentration for this complex by use of Beer s law This was done in an acidic solution and absorbencies were measured by a Spec 20 and graphed vs initial HSCN This gave molar absorbtivity the slope 4440 mol L 1 cm 1 and then concentrations were calculated by Beer s law and seen in table 4 The average Keq was found by Ice table and the average was found to be 63 2 Introduction The two main purposes of this lab are to use beer s law to measure the equilibrium concentration of a complex ion and to calculate the equilibrium constant Keq for the formation of a complex ion1 Beer s law is given by l X 1 Abs standing for absorbance X is the concentration l standing for path length and standing for the molar extinction constant Beer s law can be used to measure transmittance which was then converted to absorbance which is directly related to concentration Concentration can then be used in an ICE table Initial Change and Equilibrium Concentrations Thus the final concentrations can easily be found if the initial concentrations are known and then the equilibrium concentration is easily found In this lab the following reaction given is the basic reaction Fe3 HSCN aq H aq FeSCN2 aq 2 Or the general reaction aA bB cC dD This is given by the equilibrium expression and in this case the formation constant in the equation as follows 3 H 2 FeSCN Fe 3 K eq 4 C D c d A B b K eq 5 or the general reaction This will be used to determine the equilibrium constant by ICE table method which is measured in this lab Also certain dilutions will happen in this lab This means adding a different solution to a solution which decreases both solutions concentration The following equation can be used to determine the concentration C1 V1 C2 V2 6 Where C is concentration and V is volume 1 is the initial state and 2 is the final state Colored solutions have absorbance and thus transmittance Spectroscopy is a concept in this lab that is used in two very important tests for the determination of certain aspects of this lab A spectrophotometer is used in this lab to measure the absorbance in order to find a certain max wavelength where a solution absorbs light The max absorbance wavelength corresponds with a certain compound The diode array type of instrument measures the different absorbencies simultaneously Once this occurs Beer s law to find the concentration of certain compounds Epsilon is the constant of proportionality l is the path length and A is absorbance Different compounds absorb different wavelengths of light in the visible spectrum which correspond to the color they appear This is the color that is not absorbed The spectrophotometer measures at a certain wavelength the transmittance then calculates the absorbance from this by the equation of the negative log of percent transmittance over one hundred is equal to absorbance Absorbance can then be used to calculate the concentrations by knowing the molar extinction constant and the path length the light takes These concentrations then can be used to plot a working curve which is a graph of concentration vs absorbance graph of an unknown sample It also can be used to calculate the linearity of an instrument Le Chatelier s principle is also used in this lab which states that if either reactants or products are added to a solution the reaction will run forward or backward until it is in equilibrium again This is a general principle that is used and known when running these reactions and is used to find the Keq Which goes along with equilibrium The double arrows mean that the reaction is very able to go between both sides of the reaction in a dynamic equilibrium It will go forward and reverse for a time until the forward and reverse are in an equilibrium like a pendulum Spectroscopy is still used in many areas of science In a recent study of solid dispersions spectroscopy was used to monitor Curcumin chemical stability This aided in the determination of the inter and intra molecular strength of curcumin dispersions2 In another experiment performed spectroscopy was used to detect nuclear materials Nuclear materials have been used an can be used for very dangerous weapons and certain kinds of spectroscopy can be used to classify detect and protect from this as it was in this experiment In the future it will hopefully deter nuclear terrorism 3 Finally in another experiment Stretching of carbon nanotubes were calculated using spectroscopy This was done in a one dimensional framework4 Materials and Methods This lab had to be very exact in measurements therefore there were not many deviations from the lab manual1 The Spec 20 was turned on more than a half hour before its use Solutions were made as stated in the lab manual to exact measurements This was done by use of a Mohr pipet Some solutions were spilled in the process When creating solution 3 of the standard it was spilled and had to be restarted A portion of HSCN was contaminated with Fe NO3 3 and had to be disposed of Other than those deviations everything was measured with exact precision as far as could be measured Some cuvettes may not have been exactly three quarters full but they were close and this should not have affected the results Due to the fact that the spec 20 machines were very old and faulty when testing the absorbance the spec 20 value was taken as soon as the cuvette was placed in the machine there was no waiting for equilibrium to be reached Results The following results display the conclusions in numerical form from this lab The first table displays the amount of each solution was used in the making of the standard solutions This was done using a Mohr pipet The solutions had an orange hue and increased darkness as more HSCN was included in it It also displays the measured absorbance and the initial concentration of HSCN which was calculated and a sample calculation can be seen below the table The initial undiluted HSCN was 6x10 4 M Table 1 Standard Solutions Volumes Concentrations and Absorbencies 1 2 3 Fe NO3 3 0 2M 25mL 25mL 25mL HSCN 5mL 10mL 15mL HNO3 70mL 65mL 60mL 5 5 Initial HSCN 3 0x10 M 6 0x10 M 9 0x10 5 M Measured Abs 133 246 406 Color Light orange Orange Orange Sample Calculation for initial HSCN in the first trial M1V1 M2V2 4 25mL 20mL 55mL 1 2x10 4 M 592 Dark